This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a single component development system wherein a donor roll or belt is loaded with inductively charged toner particles, in which the toner particles are subsequently transferred to a latent image.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED array, or an original document being reproduced. By selectively discharging certain areas on the photoconductor, an electrostatic latent image is recorded on the photoconductive surface. This latent image is subsequently developed by charged toner particles supplied by the development sub-system.
Powder development systems normally fall into two classes: two components, in which the developer material is comprised of magnetic carrier granules having toner particles adhering triboelectrically thereto and a single component, which typically uses toner only. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to an intermediate or a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration. The operating latitude of a powder xerographic development system is determined to a great degree by the ease with which toner particles are supplied to an electrostatic image. Placing charge on the particles, to enable movement and imagewise development via electric fields, is most often accomplished with triboelectricity.
The triboelectric charging is obtained by either mixing the toner with larger carrier beads in a two component development system or by rubbing the toner between a blade and donor roll in a single component system.
However, all development systems which use triboelectricity to charge toner, whether they be two component (toner and carrier) or single component (toner only), have one feature in common: charges are distributed non-uniformly on the surface of the toner. This results in high electrostatic adhesion due to localized high surface charge densities on the particles. Toner adhesion, especially in the development step, is a key factor which limits performance by hindering toner release. As the toner particle size is reduced to enable higher image quality, the charge Q on a triboelectrically charged particle, and thus the removal force (F=QE) acting on the particle due to the development electric field E, will drop roughly in proportion to the particle surface area. On the other hand, the electrostatic adhesion forces for tribo-charged toner, which are dominated by charged regions on the particle at or near its points of contact with a surface, do not decrease as rapidly with decreasing size. This so-called xe2x80x9ccharge patchxe2x80x9d effect makes smaller, triboelectric charged particles much more difficult to develop and control.
Although such powder development systems have served the electrophotographic industry well over the years, there continues to be a need for improvements in toner charging since triboelectricity is not well understood and unpredictable results occur due to a strong materials sensitivity. For example, the materials sensitivity causes difficulties in identifying a triboelectrically compatible set of color toners that can be blended for custom colors. Furthermore, to enable xe2x80x9coffsetxe2x80x9d print quality with powder-based electrophotographic development systems, small toner (xcx9c5 xcexcm diameter) is desired. Although the functionality of small, triboelectrically charged toner has been demonstrated, concerns remain regarding the long-term stability and reliability of such systems.
Given that charged particle adhesion is a major limiting factor in development with dry powder, it has been a goal to identify toner charging and delivery schemes which keep toner adhesion low.
Convention single component development (SCD) systems based on induction charging within the development zone utilize a magnetic loaded toner to suppress background deposition. If with such SCD systems one attempts to suppress background deposition by using an electric field of polarity opposite to that of the image electric field (as practiced with practiced with electrophotographic systems that use a triboelectric toner charging development system), toner of opposite polarity to the image toner will be induction charged and deposited in the background regions. To circumvent this problem, the electric field in the background regions must be set to near zero. To prevent deposition of uncharged toner in the background regions, a magnetic material is included in the toner so that a magnetic force can be applied by the incorporation of magnets inside the development roll. However, the addition of magnetic material in the toner precludes bright colors since the material absorbs visible light.
An object of the present invention is to circumvent limitations associated with development systems based on triboelectrically charged toner. A non-triboelectric toner charging system is desired to enable a more stable development system with greater latitude in toner materials; and to circumvent limitations associated with conventional single component development (SCD) systems based on induction charging within the development zone.
More specifically, it is an object of the invention to describe a single component development system based on the induction charging of conductive, nonmagnetic toner prior to the development zone. The nonmagnetic toner enables bright colors for process and custom colors.
There is provided an apparatus for developing a latent image recorded on an imaging surface, including: a first developer unit for developing a portion of said latent image with a toner of custom color, said first developer including a housing defining a reservoir for storing a supply of developer material comprising conductive toner; a dispenser for dispensing toner of a first color and toner of a second color into said housing, said dispenser including means for mixing toner of said first color and toner of said second color together to form toner of said custom color; a donor member for transporting toner of said custom color on an outer surface of said donor member to a development zone; means for loading a toner layer of said custom color onto said outer surface of said donor member; and means for inductive charging said toner layer onto said outer surface of said donor member prior to the development zone to a predefine charge level; and a second developer unit for developing a remaining portion of said latent image with toner being substantial different than said toner of said custom color.